Friday, March 21, 2014

Lacus Autumni

Fresh and not-so-fresh craters on the basalt plain of Lacus Autumni, a pool of volcanic material solidified between the concentric rings of Orientale basin. Field of view from a mosaic of the left and right frames of LROC NAC observation M114498609, LRO orbit 2007, December 3, 2009; resolution 51 cm per pixel, incidence 62.57° from 48.44 km [NASA/GSFC/Arizona State University].

Though the margins of the crater are crisp and distinct, it has a lumpy appearance that is probably due to the coherence of the target material.

The ejecta of the crater has a slightly higher reflectance relative to the mare in which it is found. High-reflectance ejecta can represent recently exposed material that has not been affected by space weathering processes, called maturity rays, or material that is compositionally distinct from its surroundings, called compositional rays. Due to its crisp appearance and the predominance of blocks, we interpret this as a young, fresh crater, so the rays are likely maturity rays.

LROC WAC mosaic of Lacus Autumni, context for the LROC Featured Image. Red box outlines the the full LROC NAC field of view from LROC observation M114498609, orbit 2007, December 3, 2009. The white arrows points to the location of the fresh crater. Field of view above approximately 160 km across [NASA/GSFC/Arizona State University].

Lacus Autumni (or "Autumn Lake"), along with Lacus Veris and Mare Orientale, is a mare pond located in the northeast portion of the Orientale Basin. It lies between the Orientale inner ring (Montes Rook) and outer ring (Montes Cordillera). When craters form in thin mare sometimes high-reflectance highlands material is excavated from depth, which makes it difficult to differentiate between maturity rays and compositional rays. To resolve this issue, we can look at the composition of the material that was excavated, looking specifically at both iron and titanium maps.

Nestled in a valley between the inner and outer Orientale impact basin rings, Lacus Autumni is seen here at high relief of sunset shadows. Mosaic of LROC WAC observations from orbits 4786 through 4791, July 9, 2010; Uncropped field of view (very roughly) 205 km across, at an average resolution 68 meters, incidence 80° from 49 km. View the full-size original HERE [NASA/GSFC/Arizona State University].

If the rays are indistinguishable from the mare in which the crater formed, then we can conclude that they are highly reflective because they are young and unweathered. If the rays are composed of highlands material, the rays are likely compositional rays.

If the crater excavated highlands material from beneath the mare, then we can estimate the thickness of the mare deposit and determine just how much lava was extruded onto the surface when the mare formed. In the case of compositional rays, the morphology of the crater, such as a crisp rim or peak, is an indicator of the age of the crater.

The crater rays in this LROC Featured Image are indistinguishable from the mare in which they are found, so these are indeed maturity rays.

A well-known composite color image of the Moon's western hemisphere centered just below Lacus Autumni, northeast of Mare Orientale, captured by the Galileo spacecraft while maneuvering out of the inner solar system on its way to Jupiter, at 1735 UT December 9, 1990, from roughly 560,000 km away. The color composite was stacked from monochrome images taken through violet, red, and near-infrared filters. The Moon's nearside is to the right, the far side to the left [NASA/JPL].